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    <title>Document</title>
    <script>
        var flag=true;//falg布尔型
        var flag1=false;//falg1布尔型
        console.log(flag +1);//2  ture 参与加法运算当1来看
        console.log(flag1+1);//1   flase 参与加法运算当0来看
        // 如果一个变量声明未赋值 就是undefined来定义数据类型
        var str;
        console.log(str);
        var variable=undefined;
        console.log(variable+'pink');//variablepink
        console.log(variable+12);//NaN
        // null空值
        var space=null;
        console.log(null+'pink');//nullpink
        console.log(space+1);//1
    </script>
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